1. Field of the Invention
The present invention relates to methods of and apparatus for testing welds, and in particular to a method of and apparatus for determining the structural integrity of a weld between dissimilar metals by monitoring the emf generated at the juncture between the metals during the formation of the weld.
2. Description of the Prior Art
In the manufacture of electronic circuits, joining conductors of electricity by welding presents the problem of testing the mechanical integrity, or quality, of the weld in a nondestructive manner. Mechanical tests are often impractical because they are time consuming and can reduce the quality of the weld, or become destructive tests unless precisely controlled.
To avoid the possibility of degrading or destroying a weld between electrical conductors with a mechanical testing apparatus, various electrical testing techniques have been developed. One technique involves passing current through the weld while measuring the voltage thereacross, and then comparing the measured voltage with a voltage measured across a section of conductor without a weld when a similar magnitude current is passed therethrough. A disadvantage of this technique is the difficulty of coupling a high current to the weld while measuring a low voltage thereacross, and then comparing the two independent voltage measurements.
Another technique is to use a Kelvin bridge to measure the resistance of a length of conductor which includes the weld, which resistance is then compared with the resistance of a similar length of conductor which does not include a weld. A disadvantage of this technique is the inconvenient and time consuming operation of adjusting the Kelvin bridge for each resistance measurement. A variation of this technique is to measure the resistance across a weld while the weld is subjected to a cyclically varying pressure, it being assumed that a weld having a high degree of mechanical integrity will exhibit minimal changes in resistance in response to pressure, while a weld having less mechanical integrity will exhibit significant resistance changes in response to pressure. However, this technique requires bulky equipment for exerting pressure on the weld, and similarly requires the time consuming operation of adjusting a Kelvin bridge for each resistance measurement.
To overcome the aforementioned disadvantages in testing welds between conductors in electronic circuits, another method of testing a weld involves employing conductors of dissimilar metals and measuring the maximum emf generated across the weld bond interface during the welding operation as a result of the Seebeck effect (thermocouple). The magnitude of the generated emf is directly related to the temperature of the weld bond interface, and the maximum emf measured is therefore representative of the maximum temperature reached at the weld bond interface and allows a determination to be made whether or not a temperature sufficient to effect a weld is reached. While this technique is particularly useful in ascertaining whether or not a temperature sufficient to effect a weld is reached, it does not allow a determination of the quality of the resulting weld. That is, no indication is provided as to the mass of the weld, or whether cracking or gas venting has occurred at the weld bond interface during the formation of the weld, all of which affect the mechanical integrity of the weld.